Air separation method and apparatus

ABSTRACT

A cryogenic air separation method and apparatus in which a lower pressure distillation column is configured to receive, at successively higher locations of the lower pressure column and at successively lower temperatures, crude oxygen derived from a crude liquid oxygen stream discharged from a higher pressure column, an intermediate reflux stream and a nitrogen-rich reflux stream. All of the streams are subcooled and depressurized. The subcooling is conducted such that the intermediate reflux stream and the nitrogen-rich liquid stream cocurrently, indirectly exchange heat to a nitrogen-rich vapor stream withdrawn from the lower pressure column and the intermediate reflux stream is subcooled to a temperature between the temperatures over which the nitrogen-rich liquid stream is subcooled. Additionally, the crude liquid oxygen stream and the intermediate reflux stream can, cocurrently, indirectly exchange heat to a pressurized liquid stream used in forming an oxygen product and the nitrogen-rich vapor stream.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for separating air in which compressed and purified air is separated within an distillation column unit in which crude liquid oxygen column bottoms, an intermediate reflux stream, that can be liquid air, and a nitrogen-rich reflux stream are subcooled depressurized and introduced into the lower pressure column at successively higher locations and at successively lower temperatures. More particularly, the present invention relates to such a method and apparatus in which the subcooling of the intermediate intermediate reflux stream is enhanced to increase production of oxygen and/or argon products.

BACKGROUND OF THE INVENTION

Air is separated into its component parts by distillation that is conducted in air separation plants. Such plants employ a main air compressor to compress the air, a prepurfication unit to remove higher boiling contaminants from the air, such as carbon dioxide, water vapor and hydrocarbons, and a main heat exchanger to cool the resulting compressed and purified air to a cryogenic temperature suitable for its distillation within a distillation column unit. The distillation column unit employs a higher pressure column, a lower pressure column and optionally an argon column when argon is a desired product. Where such plants are set in an enclave, components of the air separation plants can be shared. For example, the main air compressor and purification unit may be used to compress air for two or more distillation column units.

The compressed air is introduced into the higher pressure column and is rectified into a crude liquid oxygen column bottoms, also known as kettle liquid, and a nitrogen-rich vapor column overhead. A stream of the crude liquid oxygen is introduced into the lower pressure column for further refinement within the lower pressure column to produce an oxygen-rich liquid column bottoms and a nitrogen-rich vapor column overhead of such lower pressure column. The lower pressure column operates at a lower pressure to enable the oxygen-rich liquid to condense at least part of the nitrogen-rich vapor column overhead of the higher pressure column for purposes of refluxing both columns and for production of nitrogen products from the condensate. As such, the higher and lower pressure columns are operatively associated with each other in a heat transfer relationship. Return streams of the oxygen-rich liquid, nitrogen-rich vapor from the distillation column unit can be introduced into the main heat exchanger to help cool the air. Such return streams are warmed within the main heat exchanger and are taken as oxygen and nitrogen products.

Where argon is a desired product, an argon column can be connected to the lower pressure column to rectify an argon and oxygen containing vapor stream removed from the lower pressure column. Furthermore, when an oxygen and/or a nitrogen product is desired at high pressure, potentially a supercritical pressure, a stream of the oxygen-rich liquid produced as column bottoms in the lower pressure column and/or a stream of nitrogen-rich liquid produced as condensate can be pumped and then heated in a heat exchanger to produce a high pressure vapor or a supercritical fluid. Typically, the heat exchange duty for such purposes is provided by further compressing part of the air in a booster compressor after the air has been compressed in the main air compressor. The resulting boosted pressure air stream is liquefied and the liquid air stream can be introduced into either the higher pressure column or the lower pressure column or both of such columns. In this regard, typically, the liquid air is introduced into the lower pressure column as an oxygen and nitrogen containing intermediate liquid reflux stream and such stream is subcooled along with the condensed nitrogen-rich vapor used in refluxing the lower pressure column and the crude liquid oxygen column bottoms through indirect heat exchange with a nitrogen-rich vapor stream withdrawn from the top of the lower pressure column. All of these streams after having been subcooled are depressurized by expansion valves to pressures that will enable the streams to be introduced into the lower pressure column. The depressurization also, further cools the streams. The cooling of the streams can be particular critical for the crude liquid oxygen stream which must be lowered in temperature to enable condensation of argon-rich vapor produced in the argon column for purposes of refluxing such column and taking an argon product.

In general, argon and oxygen recovery may be limited by any number of factors. For instance, argon recovery may be limited by the amount of vapor flow imparted through the base of the low pressure column by way of nitrogen condensation of the nitrogen-rich vapor from the higher pressure column. Alternatively, the upper sections of the lower pressure column may possess insufficient reflux to adequately maintain a liquid to vapor ratio sufficient to trap most of the argon/oxygen for recovery. It is this latter problem to which the subject invention is focused. Where pressurized products are produced by pumping, argon recovery can also suffer due in large part to the substantial reduction in high quality reflux flow available for the upper column. In general, 30 percent to over 35 percent of the air may be liquefied for purposes of liquid oxygen pumping. Processes which extract large fractions of pressurized nitrogen directly from the cold box will also reduce the available upper column reflux. The production of liquefied air accompanying a liquid pumped cycle (or one with a high liquid make) is typically divided between both the higher and lower pressure nitrogen rectification sections. The liquid air is only partially subcooled within the main heat exchanger prior to flashing into the column system. The resulting flash gas produced by throttling liquid air into the upper/lower pressure column results in a measureable decline in argon recovery.

In order to maximize the production of liquid oxygen and/or argon it is necessary to introduce as much liquid reflux as possible to higher regions of the lower pressure column. The introduction of such liquid reflux drives the oxygen and argon in a downward direction of the lower pressure column to increase recovery. Consequently, the liquid air, the crude liquid oxygen and the nitrogen-rich reflux are commonly all subcooled prior to being expanded and introduced into the lower pressure column to prevent the formation of vapor during the depressurization of such streams. Moreover, it is also known to subcool the crude liquid oxygen, the liquid air and the nitrogen-rich reflux to successively lower temperatures to optimize the amount of liquid reflux imparted to higher regions of the lower pressure column and therefore, further increase oxygen and/or argon recovery.

An example of such subcooling as has been discussed above can be found in U.S. Pat. No. 5,878,597. In this patent, an oxygen-rich stream taken from oxygen-rich liquid produced as column bottoms of the lower pressure column is pumped and heated to produce a pressurized oxygen product. The heat exchange duty for such heating is provided by further compressing part of the incoming compressed and purified air in a booster compressor. The heat exchange results in the liquefaction of the further compressed air and the production of a liquid air stream. The liquid air stream is introduced into the higher pressure column as an intermediate reflux stream after having been expanded in an expansion valve. Such a stream is referred to as an “intermediate reflux stream” in that it is introduced into an intermediate location of the lower pressure column, between the locations at which nitrogen-rich reflux and crude oxygen are introduced. It is to be noted that it is possible to form such an intermediate reflux stream by removal of such stream from downcoming liquid within the higher pressure column at or near the point at which liquid air is introduced. In any case, an intermediate reflux stream will contain both oxygen and nitrogen and will have a greater nitrogen fraction than that of the crude liquid oxygen. In this patent, the intermediate reflux stream is subcooled within a subcooling unit prior to being introduced into an intermediate location of the lower pressure column. The subcooling unit also serves to subcool a crude liquid oxygen stream composed of the crude liquid oxygen produced in the higher pressure column and a reflux stream composed of condensed nitrogen-rich vapor produced in the higher pressure column. The subcooling unit is designed such that the crude liquid oxygen stream, the intermediate reflux stream and the reflux stream composed of the condensed nitrogen-rich vapor are subcooled to successively lower temperatures as described above. However, such subcooling takes place over temperature ranges that are successively lower and with no overlap between the ranges. As a result of such heat exchange, the intermediate reflux stream will have a temperature that will result in the formation of meaningful flash gas/vapor upon introduction into the lower pressure column. This will adversely affect argon and oxygen recovery. Furthermore, the heat exchange itself will be inefficient in that the subcooling unit will have high temperature differences between the heating and cooling streams. In this patent, the heating stream or streams that are used for the subcooling heat exchange duty are a nitrogen product stream taken from the top of the lower pressure column and a waste nitrogen stream removed from the lower pressure column at a location below the top of such column.

As will be discussed, the present invention provides a method and apparatus for separating air in which intermediate reflux stream fed to a lower pressure column is more optimally subcooled than in the prior art by more efficiently transferring heat between heating and cooling streams during subcooling. In other words, the heating and cooling curves are closer together resulting in the intermediate reflux stream being subcooled to a lower temperature than that possible in the prior art. The optimal subcooling of the intermediate reflux stream in accordance with the present invention will decrease the degree to which flash-off vapor will be formed in the intermediate reflux stream upon introduction into the lower pressure column and therefore, increase oxygen and/or argon recovery.

SUMMARY OF THE INVENTION

The present invention provides, in one aspect, an air separation method in which air is separated in a cryogenic rectification process to at least produce an oxygen product stream. As used herein and in the claims, the term “cryogenic rectification process” means any process in which compressed and purified air is cooled to a temperature at or near its dew point and then introduced into an air separation unit comprising higher and lower pressure columns operatively associated with one another in a heat transfer relationship to distill the air into oxygen and nitrogen-rich products and also, possibly an argon product.

The cryogenic rectification process of the present invention uses a distillation column unit having a higher pressure column and a lower pressure column operatively associated with the higher pressure column in a heat transfer relationship. The lower pressure column is configured to receive, at successively higher locations of the lower pressure column, crude oxygen formed from a crude liquid oxygen stream discharged from the higher pressure column, an intermediate reflux stream and at least part of a nitrogen-rich reflux stream.

The cryogenic rectification process includes depressurizing the crude liquid oxygen stream, the intermediate reflux stream and the at least part of the nitrogen-rich reflux stream and subcooling the crude liquid oxygen stream, the intermediate reflux stream and the nitrogen-rich reflux stream to successively lower temperatures prior to depressurization and through indirect heat exchange with at least a nitrogen-rich vapor stream withdrawn from the lower pressure column. In this regard, the term “at least” is used in that the nitrogen-rich vapor stream could be a nitrogen-rich vapor stream withdraw from the top of the lower pressure column, an impure nitrogen-rich vapor stream withdrawn below the top of the lower pressure column that is also known in the art as a waste nitrogen stream or both of such streams. The intermediate reflux stream and the nitrogen-rich liquid stream cocurrently, indirectly exchange heat to at least the nitrogen-rich vapor stream over overlapping temperature ranges such that the intermediate reflux stream is subcooled to a temperature between the temperatures over which the nitrogen-rich liquid stream is subcooled. As used herein and in the claims, the term “cocurrent” means the relevant streams indirectly exchange heat at the same time to at least the nitrogen-rich vapor stream and further in the context of a heat exchange flow in the same direction.

The result of the cocurrent heat exchange is that the heating and cooling curves of the fluids being heated and cooled within a heat exchanger employed for such subcooling purposes will more closely coincide than in a prior-art device. As a result, the intermediate reflux stream will be subcooled to a greater extent than in the prior art. As mentioned above, the crude liquid oxygen stream, the intermediate reflux stream and the nitrogen-rich liquid stream all originate at a higher pressure than the lower pressure column and consequently must be depressurized to pressures suitable for their introduction into the lower pressure column. The depressurization of all of these streams also lowers their temperature, which in case of the crude liquid oxygen stream, is necessary to enable such stream to condense argon column reflux. However, as known in the art and discussed above, such depressurization also causes a significant vapor fraction to form in each of the streams. The greater subcooling of the oxygen and nitrogen containing reflux stream to a lower temperature than that possible in the prior art results in there being a significantly greater liquid fraction being present in such reflux stream to drive oxygen and argon down the column to increase their recovery over prior art air separation methods. In this regard, preferably the crude liquid oxygen stream is subcooled to a further temperature that is equal to or greater than that of the nitrogen-rich liquid stream prior to the subcooling of the nitrogen-rich liquid stream and that lies within a temperature range over which the oxygen and nitrogen containing reflux stream is subcooled. The crude liquid oxygen stream is preferably subcooled from a yet further temperature that is higher than that of the intermediate reflux stream prior to the subcooling of the intermediate reflux stream. This further narrows the gap between heating and cooling curves during the subcooling to further increase the extent to which the intermediate reflux stream is subcooled.

In a specific embodiment of the present invention, the air is separated by the cryogenic rectification process to produce at least an oxygen product stream and such that the oxygen product stream is formed by warming a pressurized liquid stream. In such embodiment, it is only required that the crude liquid oxygen stream or both the crude liquid oxygen stream and the intermediate reflux stream, cocurrently, indirectly exchange heat to the pressurized liquid stream and the at least the nitrogen-rich vapor stream. The addition of the pressurized liquid stream provides some of the heat transfer duty in subcooling at least the crude liquid oxygen stream will allow a greater proposition of the heat transfer duty to be supplied by the at least the nitrogen-rich vapor stream in subcooling the intermediate reflux stream and thereby allow such stream to be subcooled to a greater extent than in the prior art.

The distillation column unit can also be provided with an argon column. In such case, a crude argon vapor stream is withdrawn from the lower pressure column and rectified in the argon column to produce an argon-rich vapor column overhead and an oxygen containing column bottoms. An oxygen containing stream, composed of the oxygen containing column bottoms, is introduced into the lower pressure column. At least part of the crude liquid oxygen stream, after having been subcooled, is expanded and is passed in indirect heat exchange with an argon-rich vapor stream composed of the argon-rich vapor column overhead, thereby partially vaporizing the at least part of the crude liquid oxygen stream and condensing the argon-rich vapor stream to form an argon-rich liquid stream and liquid and vapor phases of the at least part of the crude liquid oxygen stream. Part of the argon-rich liquid stream is discharged as an argon product stream and another part of the argon-rich liquid stream is introduced into the argon column as an argon column reflux stream. Liquid and vapor phase streams composed of the crude liquid oxygen are introduced into the lower pressure column and form at least part of the crude oxygen introduced into the lower pressure column.

The air can be compressed and purified to form a compressed and purified air stream and part of the compressed and purified air stream is cooled through indirect heat exchange with the nitrogen-rich vapor stream and thereafter, introduced into the higher pressure column. The oxygen product stream is formed by further compressing another part of the compressed and purified air stream to form a boosted pressure compressed and purified air stream, pumping the at least part of the oxygen-rich liquid stream to form a pressurized liquid stream and warming at least part of the pressurized liquid stream through indirect heat exchange with the boosted pressure compressed and purified air stream. This produces the oxygen product from the pressurized liquid stream and a liquid air stream from at least part of the boosted pressure compressed and purified air stream. The intermediate reflux stream is composed of at least part of the liquid air stream. In the embodiment of the present invention in which the crude liquid oxygen stream or both the crude liquid oxygen stream and the intermediate reflux stream can cocurrently indirectly exchange heat to the pressurized liquid stream and the at least the nitrogen-rich vapor stream, the pressurized liquid stream and the oxygen product stream can be formed in the manner set forth directly above.

In any embodiment of the present invention, a first part of the boosted pressure compressed and purified air stream can be fully cooled and forms the liquid air stream. A second part of the boosted pressure compressed and purified air stream is partially cooled and introduced into a turboexpander to form an exhaust stream. The exhaust stream is introduced into one of the higher pressure column and the lower pressure column to impart refrigeration into the cryogenic rectification process.

In another aspect, the present invention provides an air separation apparatus. The air separation apparatus includes an air separation plant configured to separate air by cryogenic rectification to at least produce an oxygen product stream. As used herein and in the claims, the term “air separation plant” means a plant having a main heat exchanger to cool compressed and purified air to a temperature suitable for its rectification within an air separation unit having a distillation column unit including higher and lower pressure columns operatively associated in a heat transfer relationship to distill the air into oxygen and nitrogen-rich fractions and also, possibly an argon column to further distill the air into an argon fraction.

In accordance with the present invention, the air separation plant has a distillation column unit comprising a higher pressure column and a lower pressure column operatively associated with the higher pressure column in a heat transfer relationship. The lower pressure column is configured to receive, at successively higher locations of the lower pressure column, crude oxygen formed from a crude liquid stream discharged from the higher pressure column, an intermediate reflux stream and at least part of nitrogen-rich reflux stream. Also included is a set of expansion valves and a subcooling heat exchanger. As used herein and in the claims, the term, “subcooling heat exchanger” means any heat exchanger used for such subcooling purposes and may be either a separate unit or incorporated into the main heat exchanger and thereby form a part thereof.

The expansion valves are positioned to depressurize the crude liquid oxygen stream, the intermediate reflux stream and the at least part of the nitrogen-rich reflux stream. The subcooling heat exchanger is positioned upstream of the expansion valves and connected to the lower pressure column to receive at least a nitrogen-rich vapor stream from the lower pressure column. The subcooling heat exchanger is configured such that the crude liquid oxygen stream, the intermediate reflux stream and the nitrogen-rich liquid stream are subcooled to successively lower temperatures through indirect heat exchange with the at least the nitrogen-rich vapor stream. The intermediate reflux stream and the nitrogen-rich liquid stream cocurrently, indirectly exchange heat to the at least the nitrogen-rich vapor stream and the intermediate reflux stream is subcooled to a temperature between the temperatures over which the nitrogen-rich liquid stream is subcooled. Preferably, the subcooling heat exchanger can also be configured such that the crude liquid oxygen stream is subcooled to a further temperature that is equal to or greater than that of the nitrogen-rich liquid stream prior to the subcooling of the nitrogen-rich liquid stream and that lies within a temperature range over which the intermediate reflux stream is subcooled. More preferably, the subcooling heat exchanger is also configured such that the crude liquid oxygen stream is subcooled from a yet further temperature, higher than that of the intermediate reflux stream prior to the subcooling of the intermediate reflux stream.

In another specific embodiment of the present invention, the air separation plant is configured to separate air by cryogenic rectification to produce at least an oxygen product stream from a pressurized liquid stream. In such embodiment, the subcooling heat exchanger can be configured such that, at minimum, the crude liquid oxygen stream or both the crude liquid oxygen stream and the intermediate reflux stream cocurrently, indirectly exchange heat to the pressurized liquid stream and at least the nitrogen-rich vapor stream.

The distillation column unit can also be provided with an argon column. The argon column is connected to the lower pressure column such that a crude argon vapor stream is withdrawn from the lower pressure column and rectified in the argon column to produce an argon-rich vapor column overhead and an oxygen containing column bottoms and an oxygen containing stream composed of the oxygen containing column bottoms is introduced into the lower pressure column. An argon condenser is also provided and an expansion valve of the set of expansion valves is positioned between the subcooling unit and the argon condenser such that at least part of the crude liquid oxygen stream is expanded after having been subcooled. The argon condenser is connected to the argon column and to the expansion valve, and configured such that at least part of the crude liquid oxygen stream, after having been subcooled and expanded, is passed in indirect heat exchange with an argon-rich vapor stream composed of the argon-rich vapor column overhead, thereby partially vaporizing the at least part of the crude liquid oxygen stream and condensing the argon-rich vapor stream to form an argon-rich liquid stream and liquid and vapor phases of the at least part of the crude liquid oxygen stream. Part of the argon-rich liquid stream is discharged from the argon condenser to form an argon product stream and another part of the argon-rich liquid stream is introduced from the argon condenser into the argon column as an argon column reflux stream. The argon condenser is connected to the lower pressure column such that liquid and vapor phase streams composed of the liquid and vapor phases, respectively, are introduced into the lower pressure column to form at least part of the crude oxygen introduced into the lower pressure column.

The air separation plant can be provided with a main heat exchanger, a main air compressor to compress the air, a purification unit connected to the main air compressor to purify the air after having been compressed and thereby to form a compressed and purified air stream. A booster compressor is connected to the purification unit to form a further compressed and purified air stream. The main heat exchanger is connected between the purification unit and the higher pressure column and configured such that part of the compressed and purified air stream is cooled through indirect heat exchange with the nitrogen-rich vapor stream and introduced into the higher pressure column. A pump is connected to the distillation column unit to pump the at least part of the oxygen-rich liquid stream to form a pressurized liquid stream and the main heat exchanger is connected to the booster compressor and also configured to warm at least part of the pressurized liquid stream through indirect heat exchange with the boosted pressure compressed and purified air stream, thereby producing a liquid air stream from at least part of the boosted pressure compressed and purified air stream. The main heat exchanger is in flow communication with the lower pressure column and another expansion valve of the set of expansion valves is positioned between the subcooling unit and the lower pressure column such that at least part of the liquid air stream is subcooled within the subcooling heat exchanger, expanded and introduced into the lower pressure column and thereby forms the oxygen and nitrogen containing intermediate liquid reflux stream. The subcooling heat exchanger can be connected between the lower pressure column and the main heat exchanger and also configured such that the oxygen-rich liquid stream is partially warmed through indirect heat exchange with the crude liquid oxygen stream during the subcooling of the crude liquid oxygen stream. In the embodiment discussed above where the subcooling heat exchanger is configured such that the crude liquid oxygen stream or both the crude liquid oxygen stream and the intermediate reflux stream cocurrently, indirectly exchange heat to the pressurized liquid stream and at least the nitrogen-rich vapor stream, the pressurized liquid stream and the oxygen product stream can be formed in the manner set forth above.

The air separation plant can have a turboexpander connected to one of the lower pressure column and the higher pressure column such that an exhaust stream generated by the turboexpander is introduced into the one of the lower pressure column and the higher pressure column to impart refrigeration into the air separation plant. The main heat exchanger is configured such that a first part of the boosted pressure compressed and purified air stream is fully cooled and forms the liquid air stream and a second part of the boosted pressure compressed and purified air stream is partially cooled and discharged from the main heat exchanger. The turboexpander is connected to the main heat exchanger such that the second part of the boosted pressure compressed and purified air stream is expanded in the turboexpander to generate the exhaust stream.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims distinctly pointing out the subject matter that Applicant regards as his invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which:

FIG. 1 is a schematic diagram of an air separation apparatus for carrying out a method in accordance with the present invention; and

FIG. 2 is a graphical representation of an example of the temperatures along the length of a subcooling heat exchanger of the present invention.

DETAILED DESCRIPTION

With reference to the FIG. 1, an air separation apparatus 1 is illustrated that is designed to conduct a cryogenic rectification process to produce both a pressurized oxygen product and an argon product. The present invention is not, however, limited to such an apparatus and has more general application to any such apparatus that is designed to produce an oxygen product, with our without an argon product.

More specifically, in air separation apparatus 1, a feed air stream 10 is compressed by a main air compressor 12 and then purified within a purification unit 14. Main air compressor 12 can be a multi-stage machine with intercoolers between stages and an after-cooler to remove the heat of compression from the final stage. Although not illustrated, a separate after-cooler could be installed directly downstream of compressor 12. Prepurification unit 14 as well known to those skilled in the art can contain beds of adsorbent, for example alumina or molecular sieve-type adsorbent to adsorb the higher boiling impurities contained within the air and therefore feed air stream 10. For example such higher boiling impurities as well known would include water vapor and carbon dioxide that will freeze and accumulate at the low rectification temperatures contemplated by air separation apparatus 1. In addition, hydrocarbons can also be adsorbed that could collect within oxygen-rich liquids and thereby present a safety hazard.

The resulting compressed and purified air stream 16 is then divided into first and second subsidiary compressed and purified air streams 18 and 20. First subsidiary compressed and purified air stream 18 is cooled to near saturation within a main heat exchanger 22. It is to be noted that although main heat exchanger 22 is illustrated as a single unit, as would be appreciated by those skilled in the art, the exact means for cooling the air and for conducting other heat exchange operations could differ from that illustrated. Typically, the means utilized would consist of two or more heat exchangers connected in parallel and further, each of such heat exchangers could be split in segments at the warm and cold ends thereof. Furthermore, the heat exchangers could further be divided in a banked design in which the heat exchange duty required at high pressures, for example between a boosted pressure compressed and purified air stream 53 and the a first part 104 of at least part of a pressurized liquid stream 108, both to be discussed, is conducted in one or more high pressure heat exchangers. Other heat exchange duty that is to be conducted at lower pressures is conducted in a lower pressure heat exchanger, for example, that between first subsidiary compressed and purified air stream 18 and nitrogen-rich vapor stream 94, also to be discussed. All of such heat exchangers can be of plate-fin design and incorporate braised aluminum construction. Spiral wound heat exchangers are a possible alternative for the higher pressure heat exchangers.

The resulting compressed, purified and cooled stream 24 is then introduced into an distillation column unit 26 having higher and lower pressure columns 28 and 30 and an argon column 32. Specifically, compressed, purified and cooled stream 24 is introduced into the higher pressure column 28 that operates at a pressure of between about 5 and about 6 bar(a) and is so designated as “higher” in that it operates at a higher pressure than the lower pressure column 30 that is designated as “lower” in that it operates at a lower pressure than the higher pressure column 28. Higher pressure column 28 is provided with mass transfer contacting elements generally shown by reference numbers 34 and 36 that are used to contact an ascending liquid phase of the mixture to be separated, air, with a descending liquid phase. As the vapor phase ascends within the column it becomes richer in nitrogen to produce a nitrogen-rich vapor column overhead 76 and a crude liquid oxygen column bottoms 50, also known as kettle liquid, that will be further refined in the lower pressure column 30. The mass transfer elements may be comprised of structured packing, trays, random packing or a combination of such elements. Lower pressure column 30 is provided with such mass transfer elements generally indicated by reference numbers 38, 40, 42, 44 and 46 and argon column 32 is also provided by mass transfer elements generally indicated by reference number 48.

Second subsidiary compressed and purified air stream 20 is further compressed in a booster compressor 52 to produce the boosted pressure compressed and purified air stream 53 that is introduced into main heat exchanger 22. Boosted pressure compressed and purified air stream 53 constitutes between about 30 percent and about 40 percent of the total air entering the air separation apparatus 1. A first part 54 of the boosted pressure compressed and purified air stream 53 is removed from the main heat exchanger 22 after a partial traversal thereof and is expanded in an expansion turbine 56 to generate refrigeration by production of an exhaust stream 58 at a pressure of between about 1.1 and about 1.5 bar(a) that is introduced into the lower pressure column 30. Typically, first part 54 of boosted pressure compressed and purified air stream 53 constitutes between about 10 percent and about 20 percent of the boosted pressure compressed and purified air stream 53. It should be noted that the shaft work of expansion may be imparted to the compression of the expansion stream or used for purposes of compressing another process stream or generating electricity. As known in the art, refrigeration must be imparted into an air separation plant for such purposes as compensating for warm end losses in the heat exchangers, heat leakage into the plant and to produce liquids. Other means are also known in the art to produce such refrigeration such as introducing turbine exhaust into the higher pressure column, nitrogen expansion of a nitrogen-rich stream taken from the lower pressure column after the partial warming thereof as well as other expansion cycles known in the art. It is equally possible that the refrigeration be imparted from an external source as might be the case in an air separation plant enclave. A second or remaining part of the boosted pressure compressed and purified air stream 53 upon cooling within the main heat exchanger 22 forms a liquid air stream 60 that has a temperature in a range of between about 98K and about 103K. It is to be noted that the first part 54 of the boosted pressure compressed and purified air stream 53 could be produced by removing a stream from booster compressor 52 at an intermediate stage and then further compressing such stream. The second boosted pressure compressed and purified air stream 53 could then be introduced into the main heat exchanger 22 and fully traverse the same.

Liquid air stream 60 is subsequently divided into a first part 62 and a second part 64. Alternatively, the liquefied air stream may be partially depressurized by way of a dense phase expander (liquid turbine) or expansion valve. First part 62 of liquid air stream 60 is valve expanded by expansion valve 66 and introduced into higher pressure column 28 and the second part 64 forms intermediate reflux stream that after having been subcooled within subcooling heat exchanger 70 is expanded by passage through expansion valve 65 and introduced into the lower pressure column 30. It is to be noted that other means could be used for forming the intermediate reflux stream. For example, the intermediate reflux stream could be formed by valve expanding and introducing the liquid air stream 60 into the higher pressure column 28 and then forming the intermediate reflux stream from downcoming liquid removed from the higher pressure column 28 at the location thereof at which the liquid air stream 60 was introduced. In such case, the intermediate reflux stream would have a composition close to that of the incoming liquid air stream 60. Also, the intermediate reflux stream could be formed from downcoming liquid taken at a higher location of the higher pressure column 28. However, in all cases, the intermediate reflux stream will have a higher nitrogen fraction than the crude liquid oxygen column bottoms 50.

A crude liquid oxygen stream 68 composed of a crude liquid oxygen column bottoms 50, produced in the higher pressure column 28, is subcooled in a subcooling heat exchanger 70 and further refined in the lower pressure column 30 in a manner that will also be discussed hereinafter. As well known in the art, other means could be used such as integrating the subcooling function into part of the main heat exchanger 22. It is to be noted that where a separate subcooling unit is utilized, the physical position of the exchanger may necessitate a liquid pump to motivate crude liquid oxygen back to the upper column. The refinement of the crude liquid oxygen produces an oxygen rich liquid column bottoms 72 of the lower pressure column 30 that is partially vaporized in a condenser reboiler 74 in the bottom of the lower pressure column 30 against condensing a nitrogen-rich vapor column overhead stream 76 removed from the higher pressure column 28. The resulting nitrogen-rich liquid stream 78 is divided into first and second nitrogen-rich reflux streams 80 and 82 that serve as reflux to the higher pressure column 28 and the lower pressure column 30, respectively. The second nitrogen-rich reflux stream could be formed by introducing all of the nitrogen-rich liquid stream 78 into the higher pressure column 28 and then taking an impure nitrogen liquid stream from a lower location of the higher pressure column 28.

Second nitrogen-rich reflux stream 82 is subcooled within the subcooling heat exchanger 70 and is at least in part, as a reflux stream 84, valve expanded by an expansion valve 86 and introduced as reflux into the lower pressure column 30. A liquid nitrogen product stream 87 can optionally be taken after expansion in a valve 88. The subcooling heat exchange duty is provided with a nitrogen-rich vapor stream 94 that is made up of column overhead from the lower pressure column 30. After having been partially warmed within the subcooling heat exchanger 70, the nitrogen-rich vapor stream is fully warmed within main heat exchanger 22 and taken as a nitrogen product stream 96. In installations where a waste nitrogen stream is produced and taken from a lower location of the lower pressure column 30, such stream can also participate in the subcooling heat exchange duty.

As illustrated all or optionally, part of an oxygen-rich liquid stream 78, composed of the oxygen-rich liquid column bottoms 72 is pumped by a pump 100 to produce a pressurized liquid stream 102. A first part 104 of the pressurized liquid stream 102 can be heated in main heat exchanger 22 in indirect heat exchange with the first subsidiary compressed air stream 18 to produce a pressurized oxygen product stream 106. Depending upon the degree of pressurization of pressurized liquid stream 102, pressurized oxygen product stream 106 will either be a supercritical fluid or will be a high pressure vapor. Optionally, a part 108 of the pressurized liquid stream 102 can be valve expanded within an expansion valve 110 and taken as an oxygen-rich liquid product stream 112. As would be known to those skilled in the art, additionally or in lieu thereof, another component-rich liquid stream enriched in nitrogen could be used to form a pressurized product.

Argon column 32 operates at a pressure comparable with the lower pressure column 30 and typically will employ between 50 and 180 stages depending upon the amount of argon refinement that is desired. A gaseous argon and oxygen containing feed stream 114 is removed from the lower pressure column 30 at a point at which the argon concentration is at least near maximum and the argon and oxygen containing feed is rectified within the argon column 32 into an argon-rich vapor column overhead and an oxygen-rich liquid column bottoms. An argon-rich vapor stream 115, composed of column overhead produced in argon column 32, is condensed in an argon condenser 116 having a shell 117 and a core 118 to produce an argon-rich liquid stream 120. A part 122 of the argon-rich liquid stream 120 is returned to the argon column 32 as reflux stream 122 and a part 124 is valve expanded within an expansion valve 126 and taken as an argon product stream 128. Depending on the number of stages, such argon-rich product can be further processed to remove oxygen and nitrogen in a manner known in the art. For example, additional stages could be incorporated in a separate column to further refine the argon product stream 128. Alternatively, the argon product stream 128 could be further refined by a deoxo unit to remove the oxygen and a lights distillation column to remove the nitrogen.

The resulting oxygen-rich and argon-lean liquid column bottoms of the argon column 32 can taken as a stream 130, pumped by a pump 132 and then returned as an argon-lean liquid stream back 134 to the lower pressure column 30. It should be noted for control purposes a portion of the pump discharge may be optionally returned to the sump of column 32.

Crude liquid oxygen stream 68 composed of the crude liquid oxygen column bottoms 50 of the higher pressure column 28 is subcooled within subcooling heat exchanger 70, previously discussed, and then divided into first and second subsidiary crude liquid oxygen streams 138 and 140. The first subsidiary crude liquid oxygen stream 138 is valve expanded in an expansion valve 142 and introduced into a shell 117 housing the core 118 to condense the argon-rich vapor stream 115. This partially vaporizes first subsidiary crude liquid oxygen stream 138 and produces liquid and vapor phases. Liquid and vapor phase streams 146 and 148, that are composed of such liquid and vapor phases, respectively, are introduced into the lower pressure column 30 for further refinement of the crude liquid oxygen column bottoms 50. Additionally, second subsidiary crude liquid oxygen stream 140 is valve expanded in a valve 150 and then introduced into the lower pressure column 30 for further refinement.

In order to optimally subcool the intermediate reflux stream to the lower pressure column, the subcooling heat exchanger 70 is designed such that the crude liquid oxygen stream 68, the second part 64 of the liquid air stream 60, which serves as the intermediate reflux stream, and the second nitrogen-rich reflux stream 82 are all subcooled to successively lower temperatures, namely, “T2”; “T4”; and “T5”, respectively, through indirect heat exchange with the nitrogen-rich vapor stream 94. However unlike the prior art, discussed above, in which the subcooling is sequential, the subcooling heat exchanger 70 is designed in a manner such that the second part 64 of the liquid air stream 60 that serves as the intermediate reflux stream and the second nitrogen-rich reflux stream 82 cocurrently indirectly exchange heat with the nitrogen-rich vapor stream 94 and the second part 64 of the liquid air stream 60 is subcooled to a temperature “T4” which is between the temperatures over which the second nitrogen-rich reflux stream 82 is subcooled, namely, temperatures “T3” and “T5”. The effect of this, as described above, is to decrease the separation of the heating and cooling curves in the subcooling heat exchanger 70 and thereby to allow the second part 64 of the liquid air stream 60 to be subcooled to a lower temperature than would otherwise be possible then the prior art and prevent the formation of a significant vapor fraction within the second part 64 of liquid air stream 60 after expansion in expansion valve 65.

Preferably, as illustrated, the crude liquid oxygen stream 68 is subcooled from a temperature “T0” to a temperature of “T2”. Temperature “T2” is greater than or equal to the temperature of the second nitrogen-rich reflux stream 82 prior to the subcooling of such stream, namely, temperature “T3”. Further, this temperature “T2” lies within the temperature range over which the second part 64 of the liquid air stream 60 is subcooled, namely between “T1” and “T4”. This further allows heating and cooling curves within the subcooling heat exchanger 70 to be more closely matched resulting in a lower temperature of the intermediate reflux stream fed to the lower pressure column 30. Although, as illustrated, “T0” is higher than the temperature “T1”, such temperature, a possible, but less preferred embodiment is that “T0” is at a lower temperature than “T1”.

Also, as illustrated, optionally, the subcooling heat exchanger 70 can be designed such that the crude liquid oxygen stream 68 and the second part 64 of the liquid air stream 60, cocurrently, indirect exchange heat to the oxygen-rich liquid stream 78 and the nitrogen-rich vapor stream 94. In a possible embodiment of the present invention, the subcooling heat exchanger 70 could be designed such that only the crude liquid oxygen stream 68 engaged in such heat exchange. Typically, the base of the lower pressure column 42 will typically be positioned at a higher of about 70 to 100 feet above ground level while the subcooling unit is located at ground level. As a result, the oxygen-rich liquid stream 78 will effectively warm within the subcooling heat exchanger 70 and thereby decrease the degree to which such stream is required to be further cooled within the main heat exchanger 22. In this regard, it is possible to form an oxygen product stream 106 that is pressurized by the head developed due to the height difference discussed above and without any pumping of the oxygen-rich liquid stream 78. It is also to be noted that embodiments of the present invention are also possible in which only the crude liquid oxygen stream 68 or both the crude liquid oxygen stream 68 and the second part 64 of the liquid air stream 60 indirectly exchange heat to the oxygen-rich liquid stream 78 and the nitrogen-rich vapor stream 94. In a possible embodiment of the present invention where only the crude liquid oxygen stream 68 engages in the indirect heat transfer, the subcooling of second nitrogen-rich reflux stream 82 or both such stream and the second part 64 of the liquid air stream 60 could be sequential as in the prior art. In such case, the second part 64 of the liquid air stream would still be subcooled to a lower temperature than the prior art given the fact that the a portion of the subcooling requirements of the nitrogen-rich vapor stream 94 would be supplied by the oxygen-rich liquid stream 78 and more heat load would be available for other streams.

Subcooling heat exchanger 70 can be a brazed aluminum, plate-fin type of heat exchanger. The detailed design of such a heat exchanger to accomplish the heat transfer requirements of the present invention would be a matter of routine design to one skilled in the art. With reference to FIG. 2, an example of a possible operation of subcooling heat exchanger 70 is illustrated with respect to the air separation plant shown in FIG. 1 and without the heating of the oxygen-rich liquid stream 78. FIG. 2 shows a composite curve of the temperature difference within subcooling heat exchanger 70 with respect to the hot side temperature and between heating and cooling curves. As known in the art, such curves are produced through flow and enthalpy weighted averages of the various streams being heated and cooled. By inspection, there are multiple pinch point within subcooling heat exchanger 70 in this particular exemplar. However, “T4”, the temperature of the intermediate reflux stream upon subcooling would be 87.15K as opposed to ˜96 K which would typically be seen in the prior art. Process simulations have shown an increase in oxygen recovery of about 0.4 percent and an increase in argon recovery of about 1.7 percent through a reduction of upwards of 70 percent of the flash gas evolved from the depressurization of the liquid air or in other words the oxygen and nitrogen containing reflux stream after depressurization within expansion valve 65.

While the present invention has been described with reference to preferred embodiments, as would occur to those skilled in the art, numerous changes, additions and omissions could be made without departing from the spirit and scope of the invention as set forth in the appended claims. 

I claim:
 1. An air separation method comprising: separating air by a cryogenic rectification process to produce at least an oxygen product stream; the cryogenic rectification process using a distillation column unit having a higher pressure column and a lower pressure column operatively associated with the higher pressure column in a heat transfer relationship and configured to receive, at successively higher locations of the lower pressure column, crude oxygen formed from a crude liquid oxygen stream discharged from the higher pressure column, an intermediate reflux stream and at least part of a nitrogen-rich reflux stream; the cryogenic rectification process including depressurizing the crude liquid oxygen stream, the intermediate reflux stream and the at least part of the nitrogen-rich reflux stream and subcooling the crude liquid oxygen stream, the intermediate reflux stream and the nitrogen-rich reflux stream to successively lower temperatures prior to depressurization and through indirect heat exchange with at least a nitrogen-rich vapor stream withdrawn from the lower pressure column; and the intermediate reflux stream and the nitrogen-rich reflux stream cocurrently, indirectly exchanging heat to the at least the nitrogen-rich vapor stream such that the intermediate reflux stream is subcooled to a temperature between the temperatures over which the nitrogen-rich liquid stream is subcooled.
 2. An air separation method comprising: separating air by a cryogenic rectification process to produce at least an oxygen product stream and such that the oxygen product stream is formed by warming a pressurized liquid stream; the cryogenic rectification process using a distillation column unit having a higher pressure column and a lower pressure column operatively associated with the higher pressure column in a heat transfer relationship and configured to receive, at successively higher locations of the lower pressure column, crude oxygen formed from a crude liquid oxygen stream discharged from the higher pressure column, an intermediate reflux stream and at least part of a nitrogen-rich reflux stream; the cryogenic rectification process including depressurizing the crude liquid oxygen stream, the intermediate reflux stream and the at least part of the nitrogen-rich reflux stream and subcooling the crude liquid oxygen stream, the intermediate reflux stream and the nitrogen-rich reflux stream to successively lower temperatures prior to depressurization and through indirect heat exchange with at least a nitrogen-rich vapor stream withdrawn from the lower pressure column; and the crude liquid oxygen stream or both the crude liquid oxygen stream and the intermediate reflux stream cocurrently, indirectly exchanging heat to the pressurized liquid stream and the at least the nitrogen-rich vapor stream.
 3. The method of claim 1, wherein the crude liquid oxygen stream is subcooled to a further temperature that is equal to or greater than that of the nitrogen-rich reflux prior to the subcooling of the nitrogen-rich reflux stream and that lies within a temperature range over which the intermediate reflux stream is subcooled.
 4. The method of claim 3, wherein the crude liquid oxygen stream is subcooled from a yet further temperature, higher than that of the intermediate reflux stream prior to the subcooling of the intermediate reflux stream.
 5. The method of claim 1 or claim 2, wherein: the distillation column unit also has an argon column; a crude argon vapor stream is withdrawn from the lower pressure column and rectified in the argon column to produce an argon-rich vapor column overhead and an oxygen containing column bottoms; an oxygen containing stream composed of the oxygen containing column bottoms is introduced into the lower pressure column; at least part of the crude liquid oxygen stream, after having been subcooled, is expanded and is passed in indirect heat exchange with an argon-rich vapor stream composed of the argon-rich vapor column overhead, thereby partially vaporizing the at least part of the crude liquid oxygen stream and condensing the argon-rich vapor stream to form an argon-rich liquid stream and liquid and vapor phases of the at least part of the crude liquid oxygen stream; part of the argon-rich liquid stream is discharged as an argon product stream and another part of the argon-rich liquid stream is introduced into the argon column as an argon column reflux stream; and liquid and vapor phase streams composed of the crude liquid oxygen, respectively, are introduced into the lower pressure column and form at least part of the crude oxygen introduced into the lower pressure column.
 6. The air separation method of claim 1, wherein: the air is compressed and purified to form a compressed and purified air stream; part of the compressed and purified air stream is cooled through indirect heat exchange with the nitrogen-rich vapor stream and thereafter, introduced into the higher pressure column; the oxygen product stream is formed by further compressing another part of the compressed and purified air stream to form a boosted pressure compressed and purified air stream, pumping the at least part of the oxygen-rich liquid stream to form a pressurized liquid stream and warming at least part of the pressurized liquid stream through indirect heat exchange with the boosted pressure compressed and purified air stream, thereby producing the oxygen product from the pressurized liquid stream and a liquid air stream from at least part of the boosted pressure compressed and purified air stream; and the intermediate reflux stream is composed of at least part of the liquid air stream.
 7. The air separation method of claim 6, wherein the crude liquid oxygen stream or both the crude liquid oxygen stream and the intermediate reflux stream cocurrently, indirectly exchanges heat to the pressurized liquid stream and at least the nitrogen-rich vapor stream and the pressurized liquid stream.
 8. The air separation method of claim 2, wherein: the air is compressed and purified to form a compressed and purified air stream; part of the compressed and purified air stream is cooled through indirect heat exchange with the nitrogen-rich vapor stream and thereafter, introduced into the higher pressure column; the oxygen product stream is formed by further compressing another part of the compressed and purified air stream to form a boosted pressure compressed and purified air stream, pumping the at least part of the oxygen-rich liquid stream to form the pressurized liquid stream and warming at least part of the pressurized liquid stream through indirect heat exchange with the boosted pressure compressed and purified air stream, thereby producing the oxygen product from the pressurized liquid stream and a liquid air stream from at least part of the boosted pressure compressed and purified air stream; and the intermediate reflux stream is composed of at least part of the liquid air stream.
 9. The air separation method of claim 6 or claim 8, wherein: a first part of the boosted pressure compressed and purified air stream is fully cooled and forms the liquid air stream; a second part of the boosted pressure compressed and purified air stream is partially cooled and introduced into a turboexpander to form an exhaust stream; and the exhaust stream is introduced into one of the higher pressure column and the lower pressure column to impart refrigeration into the cryogenic rectification process.
 10. An air separation apparatus comprising: an air separation plant configured to separate air by cryogenic rectification to at least produce an oxygen product stream; the air separation plant having a distillation column unit comprising a higher pressure column and a lower pressure column operatively associated with the higher pressure column in a heat transfer relationship and configured to receive, at successively higher locations of the lower pressure column, crude oxygen formed from a crude liquid oxygen stream discharged from the higher pressure column, an intermediate reflux stream and at least part of nitrogen-rich reflux stream, a set of expansion valves and a subcooling heat exchanger; the expansion valves positioned to depressurize the crude liquid oxygen stream, the intermediate reflux stream and the at least part of the nitrogen-rich reflux stream; the subcooling heat exchanger positioned upstream of the expansion valves and connected to the lower pressure column to receive at least a nitrogen-rich vapor stream from the lower pressure column; and the subcooling heat exchanger configured such that the crude liquid oxygen stream, the intermediate reflux stream and the nitrogen-rich liquid stream are subcooled to successively lower temperatures through indirect heat exchange with at least the nitrogen-rich vapor stream, the intermediate reflux stream and the nitrogen-rich liquid stream cocurrently, indirectly exchange heat to the at least the nitrogen-rich vapor stream and the intermediate reflux stream is subcooled to a temperature between the temperatures over which the nitrogen-rich liquid stream is subcooled.
 11. An air separation apparatus comprising: an air separation plant configured to separate air by cryogenic rectification to at least produce an oxygen product stream from a pressurized liquid stream; the air separation plant having a distillation column unit comprising a higher pressure column and a lower pressure column operatively associated with the higher pressure column in a heat transfer relationship and configured to receive, at successively higher locations of the lower pressure column, crude oxygen formed from a crude liquid oxygen stream discharged from the higher pressure column, an intermediate reflux stream and at least part of nitrogen-rich reflux stream, a set of expansion valves and a subcooling heat exchanger; the expansion valves positioned to depressurize the crude liquid oxygen stream, the intermediate reflux stream and the at least part of the nitrogen-rich reflux stream; the subcooling heat exchanger positioned upstream of the expansion valves and connected to the lower pressure column to receive at least a nitrogen-rich vapor stream from the lower pressure column; and the subcooling heat exchanger configured such that the crude liquid oxygen stream, the intermediate reflux stream and the nitrogen-rich liquid stream are subcooled to successively lower temperatures through indirect heat exchange with the at least the nitrogen-rich vapor stream and the crude liquid oxygen stream or both the crude liquid oxygen stream and the intermediate reflux stream, cocurrently, indirectly exchanges heat to the pressurized liquid stream and the at least the nitrogen-rich vapor stream.
 12. The air separation apparatus of claim 10, wherein the subcooling heat exchanger is also configured such that the crude liquid oxygen stream is subcooled to a further temperature that is equal to or greater than that of the nitrogen-rich liquid stream prior to the subcooling of the nitrogen-rich liquid stream and that lies within a temperature range over which the intermediate reflux stream is subcooled.
 13. The air separation apparatus of claim 12, wherein the subcooling heat exchanger is also configured such that the crude liquid oxygen stream is subcooled from a yet further temperature that is higher than that of the intermediate reflux stream prior to the subcooling of the intermediate reflux stream.
 14. The air separation apparatus of claim 10 or claim 11, wherein: the distillation column unit also has an argon column; the argon column connected to the lower pressure column such that a crude argon vapor stream is withdrawn from the lower pressure column and rectified in the argon column to produce an argon-rich vapor column overhead and an oxygen containing column bottoms and an oxygen containing stream composed of the oxygen containing column bottoms is introduced into the lower pressure column; an argon condenser; an expansion valve of the set of expansion valves is positioned between the subcooling unit and the argon condenser such that at least part of the crude liquid oxygen stream is expanded after having been subcooled; the argon condenser is connected to the argon column and to the expansion valve, and configured such that at least part of the crude liquid oxygen stream, after having been subcooled and expanded, is passed in indirect heat exchange with an argon-rich vapor stream composed of the argon-rich vapor column overhead, thereby partially vaporizing the at least part of the crude liquid oxygen stream and condensing the argon-rich vapor stream to form an argon-rich liquid stream and liquid and vapor phases of the at least part of the crude liquid oxygen stream, part of the argon-rich liquid stream is discharged from the argon condenser to form an argon product stream and another part of the argon-rich liquid stream is introduced from the argon condenser into the argon column as an argon column reflux stream; and the argon condenser is connected to the lower pressure column such that liquid and vapor phase streams composed of the liquid and vapor phases, respectively, are introduced into the lower pressure column to form at least part of the crude oxygen introduced into the lower pressure column.
 15. The air separation apparatus of claim 10, wherein: the air separation plant has a main heat exchanger, a main air compressor to compress the air, a purification unit connected to the main air compressor to purify the air after having been compressed and thereby to form a compressed and purified air stream and a booster compressor connected to the purification unit to form a further compressed and purified air stream; the main heat exchanger is connected between the purification unit and the higher pressure column and configured such that part of the compressed and purified air stream is cooled through indirect heat exchange with the nitrogen-rich vapor stream and introduced into the higher pressure column; a pump is connected to the distillation column unit to pump the at least part of the oxygen-rich liquid stream to form a pressurized liquid stream and the main heat exchanger connected to the booster compressor and also configured to warm at least part of the pressurized liquid stream through indirect heat exchange with the boosted pressure compressed and purified air stream, thereby producing the oxygen product from the pressurized liquid stream and a liquid air stream from at least part of the boosted pressure compressed and purified air stream; and the main heat exchanger is in flow communication with the lower pressure column and another expansion valve of the set of expansion valves positioned between the subcooling unit and the lower pressure column such that at least part of the liquid air stream is subcooled within the subcooling heat exchanger, expanded and introduced into the lower pressure column and thereby forms the oxygen and nitrogen containing intermediate liquid reflux stream.
 16. The air separation apparatus of claim 15, wherein the subcooling heat exchanger is connected between the lower pressure column and the main heat exchanger and also configured such that the crude liquid oxygen stream or both the crude liquid oxygen stream and the intermediate reflux stream cocurrently, indirectly exchanges heat to the pressurized liquid stream and at least the nitrogen-rich vapor stream.
 17. The air separation apparatus of claim 11, wherein: the air separation plant has a main heat exchanger, a main air compressor to compress the air, a purification unit connected to the main air compressor to purify the air after having been compressed and thereby to form a compressed and purified air stream and a booster compressor connected to the purification unit to form a further compressed and purified air stream; the main heat exchanger is connected between the purification unit and the higher pressure column and configured such that part of the compressed and purified air stream is cooled through indirect heat exchange with the nitrogen-rich vapor stream and introduced into the higher pressure column; a pump is connected to the distillation column unit to pump the at least part of the oxygen-rich liquid stream to form the pressurized liquid stream and the main heat exchanger connected to the booster compressor and also configured to warm at least part of the pressurized liquid stream through indirect heat exchange with the boosted pressure compressed and purified air stream, thereby producing the oxygen product from the pressurized liquid stream and a liquid air stream from at least part of the boosted pressure compressed and purified air stream; and the main heat exchanger is in flow communication with the lower pressure column and another expansion valve of the set of expansion valves positioned between the subcooling unit and the lower pressure column such that at least part of the liquid air stream is subcooled within the subcooling heat exchanger, expanded and introduced into the lower pressure column and thereby forms the intermediate liquid reflux stream.
 18. The air separation apparatus of claim 15 or claim 17, wherein: the air separation plant has a turboexpander connected to one of the lower pressure column and the higher pressure column such that an exhaust stream generated by the turboexpander is introduced into the one of the lower pressure column and the higher pressure column to impart refrigeration into the air separation plant; and the main heat exchanger is configured such that a first part of the boosted pressure compressed and purified air stream is fully cooled and forms the liquid air stream and a second part of the boosted pressure compressed and purified air stream is partially cooled and discharged from the main heat exchanger; and the turboexpander is connected to the main heat exchanger such that the second part of the boosted pressure compressed and purified air stream is expanded in the turboexpander to generate the exhaust stream. 